An important aspect of switching electronic circuits such as class-D amplifiers and switch-mode power supplies is electro-magnetic compatibility (EMC).
One of a few measures that can be taken in a switching design to improve EMC performance is to slow down the transition speed of the output voltage node. This is commonly referred to as ‘slope control’. A well-known method of obtaining a constant slope at the output when turning on a power MOSFET is to exploit the feedback Miller-effect of the parasitic gate-drain capacitance Cgd of the power MOSFET.
FIG. 1 shows an example of power MOSFET with slope control when turning the transistor on. The gate drain capacitance Cgd and the diode are part of the transistor equivalent circuit.
In the circuit configuration shown in FIG. 1, the gate of power transistor Mpower is charged by a current source Icharge. The graph on the right-hand side of FIG. 1 shows the voltage transients of the gate voltage Vgate and output voltage Vout.
Initially, transistor Mpower is turned off and the output voltage Vout is high, i.e. near to the supply voltage. The gate voltage Vgate increases steadily until it reaches the threshold voltage VT of the power transistor Mpower. At this point in time, Mpower starts to conduct and pulls down the output node Vout. During the transition of the output node Vout, a large rate of change of voltage (dV/dt) appears across the parasitic gate-drain capacitance Cgd causing all available current Icharge to flow into Cgd.
Consequently, the gate voltage Vgate remains almost the same during the output transition yielding a characteristic pedestal in the gate voltage transient. After the transition of the output node has finished, the gate voltage is charged further until it reaches its final value. The rate of change or ‘slope’ of the output voltage Vout during the transition is controlled by the magnitude of the current Icharge and the parasitic gate-drain capacitance Cgd:
                                          ⅆ                          ⅆ              t                                ⁢                      V            out                          =                              I            charge                                C            gd                                              (        1        )            
Transition times typically are of the order of a few tens of nanoseconds. In order to obtain a constant slope during the transition, a current source Icharge is required that can be switched on and settle very fast.
This invention relates to the control of a transistor functioning as a current switch. The term “current switch” refers to an accurate current source that can be switched on and off.
The most basic circuit implementation of a current switch is shown in FIG. 2.
The circuit comprises a reference current source Iref. The current source current is mirrored to the output Iout by a current mirror circuit that comprises input and output transistors Min and Mout. The current mirror circuit is essentially turned on and off by a control transistor Mctrl, which is used to short the two current mirror transistors to turn them rapidly off.
When the control signal ctrl is high, transistor Mctrl is turned on and it shorts the gates of Min and Mout to ground. The current switch is off and the output current Iout is zero. When the control signal ctrl is low, transistor Mctrl is turned off and the reference current Iref is mirrored through Min and Mout. The output current Iout is now equal to the reference current Iref multiplied by the size ratio between Min and Mout.
Many alternative implementations are known that improve the speed and/or the accuracy of the current mirror. A number of alternatives is shown in FIG. 3.
The configuration shown in FIG. 3(a) has a unity-gain buffer A0 that decouples the gate of output transistor Mout from the input node. When Mout is very large, this improves the speed of the current mirror since the charge to load the gate of Mout is now sourced by the amplifier instead of the input reference current Iref. However, the input transistor Min and the unity-gain buffer form a feedback loop that can become unstable.
In the configuration shown in FIG. 3(b) the input transistor Min is diode-connected and the unity-gain buffer A0 copies the gate voltage of Min to the gate of Mout without forming a feedback loop. Because there is no feedback loop, this configuration will have no stability problems but it is less accurate because the finite gain and offset of amplifier A0 will introduce an error.
In the configuration shown in FIG. 3(c), the inverting input of amplifier A0 is connected to the output node. In this configuration, the feedback loop that is now formed by the amplifiers and both transistors Min and Mout forces the input voltage to be equal to the output voltage. Since now both transistors have the same gate-source voltage as well as the same drain-source voltage, the accuracy of the mirror is greatly improved. However, the stability of the feedback loop now depends on the impedances connected to the input and output.
A particular problem with the current switch shown in FIG. 2 appears when the ratio between input transistor Min and output transistor Mout is very large, e.g. 1:1000. In this case, turn-on of the current switch becomes very slow since the current that charges the gate of Mout needs to be provided by the input reference current Iref. The alternatives shown in FIG. 3 solve this problem but can have problems with stability.
Also, in order to achieve a very high speed, the amplifier used in these configurations requires a lot of power. Note that turning off a current switch is less of a problem; any desired speed can readily be achieved by appropriate dimensioning of transistor Mctrl.
There is a need for a switchable current source circuit in which turn on speed is increased without introducing possible instability.